The present invention relates to a magnetic disc device used as an external memory of an electronic computer, and in particular, to an improvement in a dehumidifying or moisture removing mechanism employed in the magnetic disc device.
FIG. 1 is a sectional view of a known magnetic disc device disclosed, for example, in U.S. Pat. No. 4,307,425. In FIG. 1, magnetic discs 1 have data written to and read from them by a magnetic head (not shown). The discs 1 are provided within an enclosure 3 which covers a base 2 on which various elements of the device are mounted. A spindle motor 4 provided on the lower surface of the base 2 rotates the magnetic discs 1. A first throttle, or restricted opening 5 provided in an opening bored in the base 2 regulates the amount of air passing therethrough, and a breathing filter 6 is provided beneath the lower surface of the base 2 to cover the first throttle 5. The breathing filter 6 adjusts a pressure difference between the inside and outside of the enclosure 3 while maintaining a dust-proof condition.
A dehumidifying mechanism or moisture removing mechanism 7 absorbs moisture within the enclosure 3 to maintain a low humidity condition. The moisture removing mechanism 7 comprises a desiccating agent 8 including silica gel or the like, a container 9 for accommodating the desiccating agent 8, a filter 10 located at an opening of the container 9 for preventing the fine powder or dust of the desiccating agent 8 from flowing out of the container 9, and a second throttle or restricted opening 11 provided in an opening bored in the base 2 for regulating the moisture absorbing rate of the desiccating agent 8. The container 9 is fixed beneath the lower surface of the base 2 so that the container 9 convers the second throttle 11.
In the magnetic disc device so configured, moisture entering the enclosure 3 through the breathing filter 6 is absorbed by the desiccating agent 8, and the relative humidity inside the enclosure 3 is low relative to the outside atmosphere. With such an arrangement, it is possible to reduce the rate of deterioration of the magnetic discs 1 and also to reduce the occurrence of adhesion between the magnetic discs 1 and the head.
One deficiency of known magnetic disc devices as described above relates to the large amount of desiccating agent necessary for reliable operation. The amount of moisture entering into the inside of the enclosure 3 depends on the configuration of the first throttle 5. If the first throttle 5 has two stages of openings each having a diameter of 0.25 mm and a length of 1 mm, the amount of water intruding into the enclosure 3 would be approximately 12 mg per day. Thus, the total amount of water intruding during a 5-year lifetime of the device would be approximately 22 g (=12 mg.times.365.times.5). Where silica gel (JIS (Japanese Industrial Standard) - Z0701), having a water absorbing ability of 25% by weight ratio, is used as a desiccating agent, the amount required will be 88 g (=22.div.0.25). Since the apparent specific weight of the silica gel is 0.9, its volume will be 98 cm.sup.3 (=88.div.0.9).
It should be noted that the amount of intruding water does not depend on the capacity of the enclosure 3, which determines the external size of the device. Hence, the volume of the desiccating agent 8 should not depend on the capacity of the enclosure 3. As a result, no matter how small the device is, in order to maintain reliability over the lifetime of the device, the same amount of the desiccating agent 8 would be necessary.
With the magnetic disc device arranged as described above, the volume of the desiccating agent 8 was large, and there was a limit as to how small the device could be made. For example, in a device having one or more thin fixed discs with a diameter of 130 mm, the external volume of the device would be approximately 1,215 cm.sup.3 (146 mm.times.41 mm.times.203 mm). In this case, the capacity of the enclosure 3 would be about 400 cm.sup.3, and further, the amount of space available for accommodating the desiccating agent 8 would be about 20 cm.sup.3. However, since 98 cm.sup.3 was required for the desiccating agent 8 using the silica gel, apparently it is impossible to accommodate the desiccating agent 8 in the enclosure 3. As a result, in the device as shown in FIG. 1, the moisture removing mechanism 7 must be provided outside the enclosure 3 to accommodate the required amount of the desiccating agent 8.
Alternatively, the desiccating agent 8 could be provided, to the extent that space was available, in the enclosure 3. However, in that case, since the necessary amount of desiccating agent would take up too much room, reliability inevitably would be sacrificed. As a result, it was difficult to provide an effective moisture removing mechanism 7 within an enclosure of a small disc drive.